The alveolar macrophage plays a pivotal role in first line host defense in the lung. The macrophage mannose receptor is highly expressed on alveolar macrophages, but not on circulating monocytes. Although originally characterized as an endocytic receptor that is rapidly recycled to the surface of endosomes, its predominant physiological role appears to be in the phagocytosis of microorganisms. The characterization of cDNAs that encode for the human and murine mannose receptors reveal a predicted polypeptide that has ectodomain that is comprised of an NH2 terminus that is cysteine rich followed by a type 2 fibronectin repeat with the most outstanding feature being a tandem array of 8 carbohydrate recognition domains which bear, on average, 30% homology to one another. A short hydrophobic transmembrane region is followed by a 45 amino acid cytoplasmic tail. Transient expression of the mannose receptor in Cos cells and competitive inhibition of man nose receptor activity in alveolar macrophages have indicated that the mannose receptor is sufficient for the binding and up take of the cyst and trophozoite forms of Pneumocystis carinii. These studies emphasize the role for the alveolar macrophage mannose receptor in first line host defense against Pneumocystis. In preliminary experiments outlined in the proposal, we have demonstrated that alveolar macrophages isolated from AIDS patients with Pneumocystis pneumonia have an almost complete downregulation in the expression of their surface mannose receptors. The ability of these cells to phagocytose Pneumocystis is almost entirely abolished although they are able to ingest latex beads and opsonized particles. Cultivation with modest upregulators of macrophage mannose receptor activity, results in a partial restoration of mannose receptor expression and P. carinii phagocytosis. We hypothesize that MR dysfunction contributes to the extracellular accumulation of P. carinii and shed cell wall products. Our goals are (l) to define the mechanisms of MR downregulation; (2) to examine if potential upregulators of MR activity, like IL-13, are able to restore MR activity; (3) determine what role, if any, individual HIV gene plays in MR downregulation. We plan to make use of the observation that Fc receptor function is preserved in HIV infected macrophages while MR is downregulated by constructing soluble mannose receptors that contain an Fc fragment in COOH terminus. We aim to establish a mannose receptor knockout mouse as the ultimate in vivo test of mannose receptor function in P. carinii infection. This project requires close collaboration with Cores 9001, 9002, and 9004 and Projects 0008 and 0010 if we are to achieve our ultimate goal of translating basic inquiry into novel therapies for P. carinii infection.